DE3876392T2 - CEMENTING OF OIL AND GAS HOLES WITH THE APPLICATION OF A CONVERTED LIQUID. - Google Patents

Abstract

Wellbores are cemented with a cement composition formed from drilling fluid occupying the wellbore by circulating a quantity of drilling fluid mixed with a cement material and a dispersant such as a sulfonated styrene copolymer with or without an organic acid into the wellbore to displace the drilling fluid and to continuously convert the drilling fluid into the cement composition. The cement composition may be recirculated through the space in the wellbore to completely evacuate any drilling fluid not converted to cement so as to minimize any faults in the cement composition occupying the wellbore. A preflush composition containing the dispersant and being rheologoically compatible with both the drilling fluid and the drilling fluid converted to cement slurry may be used as a displacement fluid.

Description

The invention relates to the cementing of oil and gas wells with a cementitious mud-converted drilling fluid that has been treated with a dispersant and is pumped into the space to be cemented in sufficient quantities to substantially replace all of the drilling mud, and which may utilize one or more cement base materials and other additives.

For oil and gas well filling, it has been proposed to convert the drilling fluid or "mud" used in drilling the hole into a cement slurry for cementing casing or tubing or otherwise stabilizing or sealing the formation near the well. U.S. Patents 3,168,139 to H.T. Kennedy et al., 3,499,491 to R.E. Wyant et al.; 3,557,876 to A. Tregasser, 3,887,009 to G.L. Miller et al., and 4,176,720 to William N. Wilson disclose compositions for cementing wells formed at least in part from drilling fluids. U.S. Patent No. 3,168,139 to H.T. Kennedy et al., 3,499,491 to R.E. Wyant et al., 3,557,876 to A. Tregasser, 3,887,009 to G.L. Miller et al., and 4,176,720 to William N. Wilson disclose compositions for cementing wells formed at least in part from drilling fluids. Patent 3,499,491 discloses a method for cementing a pipe length in a wellbore in which an aqueous drilling fluid is combined with a mixture of hydraulic cement and powdered sodium silicate glass to produce a slurry concrete that hardens at the temperature of the wellbore.

Many prior art attempts to convert drilling fluid into cementitious materials have presented certain problems because the viscosity and flocculation of the drilling fluid increase when the cementitious material is added and pumped into the wellbore. Converted drilling fluid preparations of the type proposed, for example, in US Patent 3,499,491, tend to gel and are particularly sensitive to temperature. In other words, when temperatures in the borehole exceed a predetermined value, the cement preparation tends to solidify or harden rapidly. Since the temperature conditions in a borehole are often difficult to control or predict, a reduced temperature sensitivity of the drilling fluid converted to cement is particularly desirable.

Furthermore, in the prior art methods and compositions, the replacement of the drilling fluid is incomplete due to gelation and has often resulted in poor cement bonding or incomplete filling of the casing-to-borehole annulus with a homogeneous cement. In this regard, the invention has been developed to provide improved cement compositions by converting drilling fluids and an improved method for replacing drilling fluid from the borehole and casing-to-borehole annulus so as to achieve complete filling of the space to be cemented with a homogeneous cement slurry.

The invention provides an improved method for cementing an oil or gas well, in which a drilling fluid is used by adding cementitious materials and a dispersant into a cement slurry and this cement slurry is recycled in a circuit to completely replace and convert into cement all drilling fluid in the area of the wellbore to be cemented.

Furthermore, the invention provides an improved method for converting drilling fluids into cement slurries for cementing at least portions of a wellbore. The improved cement slurry is provided using a drilling fluid which is converted by the addition of certain cementitious materials and a dispersant. This dispersant minimizes the tendency toward flocculation or gelling and the attendant viscosity increase experienced in the resulting drilling fluid-cement mixture, as well as the formation of gelled mixtures in the borehole during the replacement of unconverted drilling fluid in the area to be cemented. The dispersant preferably comprises a sulfonated styrene copolymer with or without an organic acid.

According to another aspect of the invention, sulfonated styrene-maleic anhydride (SSMA), sulfonated styrene-imide (SSI), sulfonated styrene-itaconic acid or a combination of a sulfonated styrene copolymer with one or more of the groups polyacrylates, potassium salts, phosphonates and other co- or terpolymers of partially hydrolyzed polyacrylamides can be used as a dispersant and added to the drilling fluid in a mixture with a blended cementitious material to convert the fluid into an improved cement slurry.

Upon reading the following detailed description, those skilled in the art will be able to appreciate the above-listed features of the invention and its other superior characteristics.

Drawing 1 is a schematic diagram of a wellbore and fluid circulation system for continuously mixing cement to cement the annulus between the wellbore and casing using converted drilling fluid.

Drawing 2 is a schematic diagram of a cement mixing and circulation system that can be set up for either batch mixing or continuous mixing of cement to convert drilling fluid.

The conversion of wellbore fluids into cement slurries for cementing the wellbore-casing annulus or for other wellbore cementing operations is attractive for several reasons. For example, the problem of disposal and related regulations for at least a major portion of the drilling fluid is eliminated, the conversion of drilling fluid into a cement slurry minimizes handling of drilling fluid and cement slurry, the time and expense of preparing the cement slurry is minimized, and the separation between drilling fluid and the converted cementitious slurry does not need to be maintained, particularly when considering the process and compositions of the present invention.

The conversion of drilling fluid or "mud" to a cement slurry is not without process problems and undesirable changes in the formulation. For example, the addition of cementitious materials such as mixtures of lime, silica and alumina or of lime and magnesium, silica, alumina and iron oxide or cement materials such as calcium sulfate and Portland cement to aqueous drilling fluids can significantly increase the viscosity of the fluid mixture and lead to severe flocculation. Attempts to recycle such mixtures through a wellbore can result in highly unsatisfactory circulation rates, plugging of the wellbore annulus and collapse of the earth formation near the wellbore or that the cement slurry does not mix properly. Therefore, certain dispersants such as lignite and lignosulfonates have been developed for use in drilling fluids during the drilling process.

One dispersant that has been used commercially in drilling fluid is a low molecular weight sulfonated styrene-maleic anhydride copolymer and a water-soluble salt thereof (sometimes known as "SSMA"). U.S. Patent 3,730,900 to A.C. Perricone et al. describes various drilling fluids that are treated with a dispersant to stabilize rheological and flow loss properties, particularly at high temperatures in the wellbore and in the presence of fluid contaminants. U.S. Patents 4,476,029 to A.O. Sy et al., 4,581,147 to Homer Branch III, and 4,680,128 to R.C. Portnoy, and European Patent Specification No. 0 208 536 to P. Parcevaux et al. also disclose dispersants for drilling fluids and fluid spacer compositions. Nevertheless, despite the prior art, as evidenced by the publications cited here and known to the applicant, the problem remained of how to effectively convert a drilling fluid into a suitable cement composition and replace the drilling fluid in the wellbore, including an annular region between a casing and the wellbore, in a manner that provides effective coverage of the region to be cemented with a composition that effectively bonds with the wall casing and/or plugs the well or the earth formation adjacent to the well with a barrier strong enough to prevent migration of fluids in undesirable directions and/or collapse of the walls of the wellbore or casing.

Although the addition of certain amounts of a sulfonated styrene copolymer or similar dispersant substantially reduces the tendency for flocculation or gelling of the slurry converted to a cement mix, attempts to develop a formulation with reduced viscosity and less tendency for gelling and flocculation have further led to the discovery that the addition of certain amounts of organic acids such as sodium citrate, citric acid, gluco-delta-lactone, tartaric acid and erythrobic acid and other organic acids and long chain sugars in combination with the sulfonated styrene copolymer have a synergistic effect in reducing flocculation and viscosity of the slurry converted to a cement mix. However, the addition of these organic acids can also retard the setting time of the cement slurry.

Let us briefly consider Figure 1. There is shown a system according to the invention for converting drilling fluid into a cement slurry for cementing a wall casing located in an earth formation 12 which is penetrated by a borehole 14. In the system shown in Figure 1, a casing 16 has been driven from the wellhead 18 into a portion of the formation; a second casing 20 extends into the borehole and forms an annulus 22 which may, for example, enclose the washout and void regions 24 and 26. The casing 20 extends to the wellhead 18 and is adapted to be provided with a pump 28 which circulates drilling fluid down through the interior of the casing 20 and back up through the annulus 22 to a return line 30. Drilling fluid is directed through the return line 30 to a storage tank or pit 32 and then returned to the pump 28 through a pump 33 and a line 34 during normal drilling operations. Conventional For the sake of clarity, drilling fluid handling equipment such as shale shakers, sand separators and related equipment are not included in Drawings 1 and 2.

One method of converting a drilling fluid into a cementitious slurry suitable for wells requiring relatively large amounts of cement, according to the present invention, is to provide a premixed quantity of dry mixed cementitious materials in a suitable storage device 36 from which they are introduced into a mixer 38 of a commercially available type. There, the dry cementitious materials are mixed with drilling fluid which is passed through a line 39 and the mixer 38 to the pump 28. Valves 40, 41 and 42 serve to control the fluid flow path during the conversion process. The materials fed to the mixer 38 are described herein with respect to various examples of converting drilling fluid into a cementitious slurry according to the present invention. Suitable means for adding water, not shown here, should also be provided. In many cases, and this is preferred, water and the dispersant are added to the fluid before the other materials.

Figure 2 shows a system which offers greater flexibility in the mixing process of the invention. The storage device 36 delivers predetermined quantities of dry mixed cement material of the type described above into the mixer 38; the batch mixing takes place in one or more tanks or pits 44 into which drilling fluid from the tank 32 and pump 33 has been fed through line 46. The drilling fluid in the tank 44 is pumped by a pump 48 and valves 50 and 51 is circulated through the mixer 38 over and over until the proper mix and density is achieved; when this is done, valves 50 and 52 are adjusted to direct the cement slurry through a line 54 to the pump 28. Of course, during normal drilling operations the drilling fluid will be circulated through line 30, tank 32, pump 33 and lines 53 and 54 to the pump 28. The cementitious slurry may be circulated back and forth between the wellbore 14 and the tank 44 through a connecting line having a valve 45 therebetween. The system shown in Fig. 2 may be used to continuously supply a cementitious slurry to the pump 28 by closing valve 51 and opening valve 55.

Accordingly, in the systems shown in Figs. 1 and 2, drilling fluid is readily converted into a cement slurry by either continuous mixing or batch mixing. Certain cement slurry compositions referred to herein, for example, may be stored in a plurality of tanks 44 for a relatively long period of time prior to being introduced into the wellbore. By continuously mixing or batch mixing the cemented drilling fluid, only a small amount, if any, of drilling fluid remains for disposal and all of the drilling fluid in the wellbore annulus 22 is eventually replaced with a cement composition that meets the requirements for cementing the casing in the wellbore 14 and otherwise treating the formation 12 in the desired manner. In other words, the invention converts most or all of the drilling fluid that was present in the wellbore during the drilling operation into a hardenable cement composition which is then pumped back into the annular space in the wellbore to seal it. As a result, There is no need to dispose of the fluid, which would normally be the case.

As mentioned above, water, the dispersant and other additives can be mixed into the fluid before dry materials are added in bulk.

It is believed that the improved cement composition and method for cementing a well in an earth formation can be made by converting the drilling fluid according to the invention with compositions and methods generally similar to those described herein. Water-based drilling fluids having densities of about 1.08 to 21.57 kg/l (9.0 pounds per gallon [ppg] to 18 ppg) can be converted to cement and circulated through a wellbore such as well 14 by adding from one to one hundred percent (1-100%), preferably zero to fifty percent (0-50%), water based on the original drilling fluid volume together with a dispersant comprising a sulfonated styrene copolymer in the range of 0.0014 to 0.0285 kg/l [0.50 to 10.0 pounds per original barrel based on a 42-gallon barrel (hereinafter "ppb")], and preferably less than 0.0143 kg/l (5.0 ppb). By adding the dispersant during the conversion of drilling fluid to a cement slurry, a surprising improvement in the mixing of the cement material with the drilling fluid has been achieved. One source and specification of the sulfonated styrene copolymer may be a composition comprising a low molecular weight sulfonated styrene-maleic anhydride copolymer (SSMA) available under the trade name NARLEX D-72 from National Starch and Chemical Corporation, Bridgewater, New Jersey. The dispersant may be premixed with dry cement material and other additives as listed herein and stored, for example, in the storage device 36; it may also be mixed into the drilling fluid while water is added for dilution. In addition, the dispersant may also contain selected amounts of sulfonated styrene-imide copolymer (e.g., the copolymer of sulfonated styrene and N-phenylmaleimide), sulfonated styrene-itaconic acid copolymer, or a combination of sulfonated styrene copolymer with one or more compounds selected from the group consisting of polyacrylates (i.e., polymers and copolymers of esters of acrylic acid and derivatives of acrylic acid such as methacrylic acid), partially hydrolyzed co- or terpolymers of acrylamide and potassium salts, and phosphates of these partially hydrolyzed co- and terpolymers. In addition, it is believed that monomers such as maleic anhydride, maleimide and dimethyl maleate can be added in combination with the selected polymer.

Simultaneously with or after the addition of the dispersant and water for dilution, Portland cement may also be added to the previous drilling fluid in concentrations ranging from 0.285 to 1.17 kg/l (100 ppb to 600 ppb). Hydration rate control preparations such as calcium sulfate may be added to the drilling fluid in the range of 0.0285 to 0.285 kg/l (10.0 ppb to 100.0 ppb). In addition, various selected additives such as cure retarders, accelerators as well as flow loss control compositions such as inorganic salts, calcium aluminate, lignosulfonates with or without organic acids and polymers such as hydroxyethyl cellulose (HEC), carboxymethyl hydroxyethyl cellulose (CMHEC) 2-acrylamide-2-methylpropanesulfonic acid (AMPS) and acrylic acids may be premixed with the other materials. The above-described formulations may be further modified by the addition of silica sand in an amount of up to one hundred percent by weight (100%) of the Portland cement fraction of the cement slurry to increase the high temperature stability of the formulation. Furthermore, other cementing media such as calcium aluminates and calcium sulfate described above may be added to the mixture to control the thickening time of the slurry, the rate of strength development and the overall compressive strength by varying the ratios of these substances in the mixture.

In developing the invention, a wellbore was originally cemented using a cement material of the type described in U.S. Patent 3,499,491 and commercially available under the name "C-mix". This cement composition was developed for use in converting drilling fluids into cement slurries. However, experience with this particular type of cement composition shows that the drilling fluid still gels abnormally when the dry mixed cement material ("C-mix") is added. Therefore, in developing this invention, it was decided to test this cement composition with the addition of a dispersant in the form of SSMA using a water-based lignosulfate drilling mud having a density of about 1.47 kg/l (12.3 ppg).

A well was cemented with a 12.7 cm (5.0 inch) diameter casing, with the cement initially placed at a depth of 3048 m (10,000 feet) and at a specified bottom hole temperature of 74ºC (166ºF). A batch of 37,680 l (237 barrels) of cement slurry was mixed and introduced at a pumping rate of about 11 to 12 l/sec [4.0 to 4.5 barrels per minute (bpm)] with a pumping pressure of less than about 3450 kPa (500 psig). It was found that the cement slurry was still particularly temperature sensitive, as shown by the thickening times shown in Table I below.

In addition to the formulation and thickening times, Table I also shows the compressive strength at downhole temperatures and certain rheological parameters at the temperatures specified for the drilling fluid or the "mud" alone, and the mud-to-cement mixture (MTC) formed by the 0.556 kg/L (195 ppb) "C-mix" cement mix. The data shown for the various speeds in revolutions per minute, not yet processed, were obtained by using a rotational viscometer to determine the shear stress and shear rate in accordance with API Specification No. 10. The rheological parameters listed in the tables below, including plastic viscosity (PV) in centipoise and yield strength in pounds per 100 square feet (lbs/100 ft.²) [g/m³], were measured with a fixed sleeve rotating grinding wheel viscometer using a #1 spring and a #1 grinding wheel and sleeve such as the Chan 35 type manufactured by EG & G. Chandler Engineering, Tulsa, Oklahoma. Without the increased dispersion provided by SSMA, the 0.556 kg/L (195 ppb) "C-mix" slurry was too viscous for the measuring range of the apparatus in the existing equipment. The well was cemented using a batch process generally corresponding to the setup shown in Fig. 2 of the drawings and following the general procedure of the batch mix described therein. Formulation amounts are based on "original barrels" of drilling fluid. The resulting density or the final density of the drilling fluid converted into a cement mixture was approximately 0.0396 kg/l (13.9 ppg). TABLE I Formulation: Lignosulfonate sludge Water Portland cement Type I Sodium silicate RW clay Sodium ash anhydrous Thickening times: Compressive strength: Rheological properties: Sludge * YP = yield point

The cement mixture was circulated through the well at a rate of approximately 11 l/s (4.0 bpm) for two hours. The lower 305 m (1000 feet) were then cemented with a conventional cement wash tailings slurry. Although the cement slurry [37680 l (237 barrels)] was mixed in the batch and not pumped into the well for the 36-hour period, the mixture was still pumpable when pumped into the well after cooling to a temperature of approximately 32ºC (90ºF).

Because of the temperature sensitivity of the mixture using the "C-mix" slurry shown in Table I and because of a very small "micro" ring that had formed at the casing-cement interface, the alternative cement mixture described below was developed to fill a wellbore with a nominal diameter of 21.6 cm (8.50 inches) from a depth of 2652 m (8700 feet). A batch of 127,190 l (800 barrels) of cement slurry using a lignosulfate slurry, water, SSMA and a lignosulfate retarder available under the trade name WR-15 from the Western Company of North America was mixed with a Class H Portland cement (API grade) together with a stabilizing agent in the form of calcium sulfate hemihydrate in the amounts shown in Table I below. The density was controlled by the addition of a quantity of hollow pozzolana balls or "Cenospheres". A relatively light cement slurry having a density of 1.35 kg/l (11.3 ppg) was formed and found suitable for storage since it had a stated thickening time of about 73 hours at 88ºC (190ºF) (stated times based on a 24 hour day) and was found suitable for batch mixing and relatively long term storage. The rheological properties of this formulation, as recorded in Table II, provided a surprisingly easy-to-pump slurry which was prepared in a batch process and then introduced into the wellbore and passed through the full circuit (recirculation) once at a rate of 26.5 l/s (10 bpm). During recirculation using a system similar to that shown in Fig. 2, the cement slurry was pumped through the circuit in the conventional manner as if it were drilling fluid. TABLE II Formulation: Lignosulfonate slurry Water Portland cement Class H "Cenospheres" Calcium sulfate Thickening time: Compressive strength: Rheological properties: Sludge * YP = yield point

Table III shows the formation of a cement slurry using converted drilling fluid in the form of a lignosulfate mud mixed with a conventional continuous mixing process and apparatus similar to that shown in Fig. 2. A borehole of nominal diameter 21.6 cm (8.50 inches) was cemented with a mixture of the formulation shown in Table III, with a total of 143088 l (900 barrels) of cement slurry being mixed and completely replacing the drilling fluid in the borehole and with the mixture being passed through two full circuits at a rate of 21.2 to 22.5 l/s (8.0 to 8.5 bpm). TABLE III Formulation: Lignosulfonate slurry Water Portland cement Class H 100 mesh Silica sand Calcium sulfate Thickening time: at 89ºC (192ºF): still in the flow state when pulled after two days Compressive strength: Rheological properties: Sludge * YP = yield point

Table IV provides the data on a continuously mixed cement slurry placed at a depth of 10,220 feet (3,109 m) to cement the wellbore and two 2.875 inch (7.30 cm) diameter pipe sections in open hole. The formulation shown in Table IV produced a cement slurry with a density of 15.8 ppg (1.89 kg/l) and also used a retarder formulation sold commercially by Western Company of North America under the trademark WR-6 in the formulation. Drilling fluid was completely displaced from the wellbore and the drilling fluid converted to cement slurry was passed through an additional 50% of the drained wellbore space.

The circulation rate was about 12 l/s [4.5 barrels per minute (bpm)]. TABLE IV Formulation: Lignosulfonate slurry Water Portland cement Class H 100 mesh Silica sand Calcium sulfate Thickening time: Compressive strength: Rheological properties: Sludge * YP = yield point

Table V provides further data for a cement slurry converted from a lignosulphate drilling fluid having a density of 1.86 kg/l (15.5 ppg) prepared on a continuous mixing basis using a system similar to that in Figure 2, whereby [the drilling fluid] was completely replaced and the cement slurry was passed through the entire circuit once. TABLE V Formulation: Lignosulfonate slurry Water Portland cement Class H 100 mesh Silica sand Calcium sulfate Thickening time: Compressive strength: Rheological properties: Sludge * YP = yield point

The API fluid loss rate (Specification 10) at 88ºC (190ºF) and a pressure differential of 6900 kPa (1000 psi) was 56 cc/30 min. through a 325 mesh screen.

Tables VI, VII and VIII show the results of laboratory tests on drilling fluids converted to cement preparations (MTC) using SSMA as a dispersant. In the laboratory tests shown in Table VI, salt water drilling fluid or mud was used. The density of the salt mud was primarily due to the salinity of the water in the fluid sample used to prepare a 1.53 kg/L (12.8 ppg) cement preparation.

In the examples shown in the tables below, the amount of dispersant could be increased in at least some cases to provide lower viscosities. The amounts listed were determined with regard to economics and the thickening time of the slurry. In addition, the fluid loss rate of the cement slurries according to the invention was also lower than conventional cement slurries in a desired area. TABLE VI Formulation: Salt Sludge Water Calcium Sulphate Cement Type I Thickening Time: Compressive Strength: Rheological properties: * YP = yield point

Table VII shows the development of cement strength with different cement loadings and the effectiveness of a retarder (WR-15). A cement preparation with a density of 1.58 kg/l (13.2 ppg) was formulated. TABLE VII Formulation: Lignosulphate sludge Water Calcium sulphate Portland cement Class H Silica sand Thickening times: Without retarder

The compressive strength (CS) was determined using the above formulation and with sand as a constant of 35 wt% of the cement as given below for different cement quantities: Mixture Cement Sand Density Days

The API fluid loss rate at 88ºC (190ºF) and a pressure differential of 6900 kPa (1000 psi) was 112 cc/30 min. through a 325 mesh screen.

Table VIII provides the formulation data, thickening time and compressive strength of a lignosulfate slurry converted to cement with a density of 1.90 kg/l (15.9 ppg) and shows that the stability of the cement slurry was quite good after about two months at very high temperatures. Laboratory tested cubes showed no cracks or other signs of degradation after exposure to a curing temperature of 300ºF. TABLE VIII Formulation: Lignosulfate slurry Water Kerosene Portland cement Class H Silica sand Calcium sulfate Thickening time at 121ºC (250ºF): Compressive strength:

The foregoing examples demonstrate that an improved cement composition and method is provided for cementing oil and gas wells and similar subterranean cavities or voids where the drilling fluid must be displaced and replaced by a cement material having the required strength. The cement slurry may also be recycled to ensure that the drilling fluid has been completely replaced by a material that will solidify to provide the required compressive strength.

As mentioned above, tests with water-based drilling muds using cement of class A at a ratio of 113.5 kg (250 lbs) of cement per barrel of original drilling fluid and with the addition of a dispersant containing SSMA in the range of 0.0143 to 0.0171 kg/L (5.0 ppb to 6.0 ppb), there is a limit to the viscosity reduction and antiflocculation properties. Tests with the same mud converted to a cement formulation with the addition of citric acid as a dispersant also showed some antiflocculation and viscosity reduction properties. However, the addition of citric acid and SSMA at levels of approximately 0.0114 kg/L (4.0 ppb) SSMA and 0.00285 to 0.0057 kg/L (1.0 to 2.0 ppb) citric acid resulted in superior flocculation inhibition and viscosity reduction properties, indicating the synergistic effect of a dispersant using a combination of citric acid and SSMA. It is expected that sodium citrate, gluco-delta-lactone, tartaric acid and erythrobic acid would give similar results. Table IX shows rheological data for mixtures using selected dispersants and containing the indicated dispersants and cement formulations. The formulations are based on a polymer drilling mud with a density of 1.12 kg/l (9.35 ppg) and a temperature of 27ºC (80ºF). TABLE IX Formulation Cement Class H Citric Acid Cement Class G *YP = yield point

Preparations 3) and 5) showed high gelling properties, while preparations 4) and 6) to 8) mixed well.

Table X provides data for cement preparations using the same slurry as for the example in Table IX, but which also benefit from the use of a partially hydrolyzed polyacrylamide thinner (Thin-X Thinner available from Magcobar-IMCO, Houston, Texas) in combination with the SSMA. TABLE X Formulation Cement Class H Water Thin-X

The preparations in Examples 1) and 3) mixed well, but those in Example 2) did not.

From the foregoing, it can be seen that an improved composition and method for cementing wells has been developed which comprises converting a drilling fluid or "drilling mud" into a cement composition by adding to said drilling fluid one or more cementing materials and a dispersant and passing the fluid to be converted into a cement composition back through the well to completely displace the drilling fluid or convert it into a cementing material which solidifies and provides a suitable means for sealing the well around a casing or other tubular structure.

A preferred embodiment of the invention involves preparing the cement composition in a batch process until all of the fluid required for the cementing operation or for disposal of the fluid has been converted. Alternatively, the materials added to the drilling fluid for conversion to cement may be added continuously while the drilling fluid stream is being circulated from or to the wellbore. The process of recycling the cemented fluid in a volume range of 10 to 1000% of the displacement volume of the wellbore cavity substantially ensures that all of the drilling fluid has been removed from the wellbore and that washouts, voids or other imperfections in the cement mantle or annulus are minimized.

A desirable cement composition according to the invention allows for complete circulation of the fluid from the well and replacement with the actual cement composition. Since this circulation normally involves two, but possibly up to ten, complete displacements of the system volume, which includes the well, the mixing tanks or pits and all connecting lines, it is desirable that the preparation does not begin to set or thicken before the cycle is complete. In this regard, it has been concluded that one or more cement materials with or without retarders should be used to control the rate of hydration or the onset of thickening before which an insignificant change in the rheological properties of the preparation occurs during mixing and circulation. In addition, the fluid loss properties are in a desirable range similar to the base drilling fluid.

Although preferred embodiments of the invention have been described in some detail herein, various additions or changes may be made without departing from the scope of the appended claims.

Claims (12)

1. A method of cementing a borehole penetrating an earth formation into which a conduit extends, the borehole having a space occupied by a fluid composition, the method comprising replacing the fluid composition with a cement composition to cement the space to form a seal between remote points in the formation and causing or allowing the cement composition to harden in the space, characterized in that the cement composition is prepared by adding cement material and a dispersant to an amount of the fluid composition having the same or substantially the same composition as the fluid composition. 2. A method according to claim 1, wherein a fluid medium having the composition of the fluid preparation is circulated in a circuit enclosing the space and cement material and a dispersant are added to an amount of the circulated preparation to form a hardenable cement preparation. 3. A process according to claim 1 or claim 2, wherein the dispersant comprises a sulfonated styrene copolymer selected from sulfonated styrene-maleic anhydride copolymers, sulfonated styrene-imide copolymers and sulfonated styrene copolymers in combination with a polyacrylate, a partially hydrolyzed copolymer or terpolymer of acrylamide or a potassium salt or phosphonate of such a partially hydrolyzed copolymer or terpolymer. 4. A process according to claim 1 or claim 2, wherein the dispersant comprises a low molecular weight sulfonated styrene-maleic anhydride copolymer. 5. A process according to any one of claims 1 to 4, wherein the dispersant comprises a sulfonated styrene-maleic anhydride copolymer and the curable cement composition further includes at least one compound selected from citric acid, sodium citrate, gluco-delta-lactone, tartaric acid and erythrobic acid. 6. A process according to any one of claims 1 to 5, wherein the dispersant is added to the fluid preparation together with cement material at a rate such that gelling of the fluid preparation is minimized upon addition of the cement material to the fluid preparation. 7. A process according to any one of claims 1 to 6, wherein the dispersant is added to the fluid preparation in an amount between 0.00143 and 0.0285 kg/l (0.5 ppb and 10.0 ppb) of the fluid preparation and the cement material is added to the fluid preparation in an amount of 0.285 to 1.71 kg/l (100 ppb to 600 ppb) of the fluid preparation. 8. A method according to any one of claims 1 to 7, wherein the dispersant is mixed with the cement material prior to adding the cement material to the fluid preparation. 9. A process according to any one of claims 1 to 8, wherein the cement preparation is added with a hydration rate adjusting agent in an amount of 0.0285 to 0.285 kg/l (10.0 ppb to 100.0 ppb) selected from calcium sulfate and calcium aluminate. 10. A method according to any one of claims 1 to 8, which includes the following step: Adding at least two cement materials to the fluid composition to control the hydration rate of the cement composition, selected from Portland cement, calcium sulfate and calcium aluminate. 11. A method according to any one of claims 1 to 10, which includes the following additional step: Removing drilling fluid from the space with a pre-flushing preparation comprising water and a sulfonated styrene copolymer to form a rheologically compatible material for removing the drilling fluid prior to filling the borehole with the cement preparation. 12. A process according to any one of claims 1 to 11, wherein the cement preparation comprises: - a quantity of water-based drilling fluid; - Portland cement in concentration ranges from 0.285 to 1.71 kg/l (100 pounds per original 42 US gallon barrel of drilling fluid (ppb) to about 600 ppb); - a dispersant comprising a low molecular weight sulfonated styrene-maleic anhydride copolymer in an amount of less than about 0.0143 kg/l (5.0 ppb); - calcium sulphate hemihydrate in an amount ranging from about 0.0285 to 0.285 kg/l (10.0 ppb to 100 ppb); and - finely ground silicon dioxide.